You are given 4 cards, two face up and two face down; say, 3,8, Red, Brown, and asked which card(s) must be turned over in order to test the truth of the proposition that if a card shows an even number on one face, then its opposite face is Red?

Apparently only a small percentage of those tested give the 'correct' solution - although the 'great majority of subjects agree with the logic of the solution, once it is explained to them'.

The 'correct' solution is given in the article, but it is worth attempting to solve it first, if you are not already familiar with it.

This is one of a family of interesting problems sometimes presented as 'casual' puzzles, but which are in fact highly dependent on the exact wording of the problem, and indeed on a particular interpretation of it.
The difficulty here is that some of the words from the canonical problem are deliberately omitted, leading to a different result than a reader already familiar with the test may suspect.
In fact I found this exact omission 'explanation' of the original problem I found on the Internet.